Method of testing transformer core losses, giving sine-wave results on commercial circuits

It is well known that if the wave-shape or the frequency varies from the normal, the loss in a transformer core at normal voltage varies also. It is obvious, however, that the normal loss, that is, the loss with the sine-wave shape and the normal frequency of, say, 60 cycles, can be obtained under the abnormal conditions of wave shape and frequency by either raising or lowering the voltage. What is needed then, is an indicator which will enable the voltage to be so adjusted that the loss in a transfomer core as measured upon an ordinary wattmeter will be the same as it would be at normal voltage on a circuit having a sine-wave shape of normal frequency.